The present invention is directed to methods for performing microfabrication of semiconductor based logic, memory and optoelectronic devices and micromechanical systems. In particular, the present invention is directed to microfabrication using anisotropic etching.
Microfabrication techniques are used, for example, to manufacture semiconductor-based logic, memory and optoelectronic devices, and microelectromechanical (MEMS) systems. There has been an ongoing trend toward miniaturization and optimization of these devices such that ever more stringent control is required over their geometries. One method widely applied to achieve these geometries is a process where a lithographic mask is generated on a surface and then features are etched into the surface of the underlying substrate. In order to get a faithful replica of the mask to be etched in the underlying layer, the etching process must have a high degree of anisotropy. That is, the etching rate perpendicular to the surface, as opposed to other directions relative to the surface, must be predominant. Chemical etching is undesirable since it tends to be mostly isotropic (i.e., identical in all directions) because most chemical etch rates are typically independent of direction and solely depend on diffusion.
Anisotropic etching is achieved in practice, e.g., by using reactive ion-assisted plasma etching, where active chemical etchant species (e.g., fluorine atoms) are generated in a plasma reactor along with reactive ions that are directed to strike the etch substrate perpendicular to its surface. In order to achieve anisotropy, a chemical composition is achieved in the plasma that allows the side walls of the feature to be protected from purely chemical etching by the deposition of a protective film. This film must be susceptible to being removed by the reactive ions that will be impinging primarily on the bottom surface of the trench being etched. The exposed substrate at the bottom of the feature is thus subjected to a combination of chemical and ion etching whereas the side walls are essentially untouched.
In order to achieve the desired results, the chemical composition of the plasma must be balanced correctly. It has been observed that better anisotropic etching can be achieved by balancing the formation of fluoro-organic polymeric deposits for side wall protection against the generation of the reactive etchant species that can remove exposed substrate. It has been observed that this balance may be obtained with fluorocarbons having reduced F to C ratios (hereinafter, F:C). For example, C4F8 (F:C=2) is a better anisotropic etch gas than C2F6 (F:C=3). Even lower F:C ratio gases have recently attracted attention for this application (e.g., C4F6, F:C=1.5 and C5F8; F:C=1.6). This trend has been described extensively in published literature. For example, see E. Stoffels, et al., xe2x80x9cPolymerization in Fluorocarbon Plasmas,xe2x80x9d Purazuma, Kaku Yugo Gakkaishi, 1999, 75, pgs. 800-12. For future applications, plasma sources with even lower F:C ratios are desired.
It is known that when cyanuric fluoride (C3N3F3) and other fluorinated aromatic amines are decomposed in a plasma, there is a substantial tendency to form polymeric deposits. This property was used to advantage in a plasma polymerization process described by Munro, et al. in Munro, et al., xe2x80x9cAn ESCA Study of the Inductively Coupled RF Plasma Polymerization of 2, 4, 6-Trifluoro-1,3,5-Triazine,xe2x80x9d J. Polym. Sci.: Polym. Chem. Ed. 1984, V. 22, pgs. 2661-66. This polymerization process was being studied in the interest of generating thin films for their own sake and not as a means of enhancing selectivity or profile control in a reactive ion etching application. The polymers deposited by Munro, et al. were composed of fluorine carbon and nitrogen, unlike conventional fluorocarbon based etching gases, which generate only carbon and fluorine-containing deposits.
In the prior art, anisotropic etch gases principally focused on etching silicon oxide from silicon substrates when masked with a patterned and developed polymeric photoresist. These gases generally were comprised of fluorocarbons and hydrofluorocarbons. Enhanced selectivity, and profile control were obtained by using source species having reduced F:C ratios and hence having a greater propensity to polymerize. In some cases, additive gases (e.g. carbon monoxide) are required to improve the etch selectivity, presumably by abstracting active fluorine-containing species formed in the plasma reactor. It is preferable to have a single-source compound for these processes in some cases.
In the prior art, U.S. Pat. No. 6,174,451(Hung et al.) is directed to an oxide etch process using one of three unsaturated 3- and 4-carbon fluorocarbons, specifically, hexafluorobutadiene, pentafluoropropylene, or trifluoropropyne. The unsaturated hydrofluorocarbon, together with argon, is excited into a high-density plasma in a reactor which inductively couples plasma source power into the reactor and RF biases the pedestal a pedestal electrode supporting the wafer being etched.
U.S. Pat. No. 5,843,847 (Pu et al.) is directed to a method for etching a dielectric layer on a substrate with high etching selectivity (i.e. the rate of etching of the dielectric layer to the rate of etching of the overlying resist layer or the underlying silicon, polysilicon, titanium silicide, or titanium nitride layer), low etch rate microloading, and high etch rates. Here, a substrate is placed in a process zone and a plasma is formed from the process gas in the process zone. The process gas comprises (i) fluorocarbon gas for etching the dielectric layer and for forming passivating deposits on the substrate, (ii) carbon-oxygen gas to enhance formation of passivating deposits on the substrate, and (iii) nitrogen containing gas for removing passivating deposits formed on the substrate.
U.S. Pat. No. 5,814,563 (Ding et al.) is also directed to a method for etching a dielectric layer on a substrate with high etching selectivity, low etch rate microloading, and high etch rates. The method here uses fluorohydrocarbon gas, NH3-generating gas having a liquefaction temperature from about xe2x88x9260 degrees Celsius to about 20 degrees Celsius, and carbon-oxygen gas. In the method, a substrate having a dielectric layer with resist material thereon is placed in a process zone and a process gas is introduced into the process zone. The substrate is maintained at about +/xe2x88x9250 degrees Celsius of the liquefaction temperature. A plasma is formed from the process gas to etch the dielectric layer on the substrate at an etch rate of greater than 600 nm/minute and an etching selectivity ratio for etching dielectric relative to the underlying polysilicon of substantially ∞:1.
U.S. Pat. No. 5,770,098 (Araki et al.) is directed to an etching process for a semiconductor wafer where the process includes the steps of placing the object in a vacuum processing chamber, introducing an etching gas into the vacuum processing chamber, and applying electrical power to a pair of electrodes within the chamber by a high-frequency electrical power source. A mixed gas of carbon monoxide and a gas which does not contain hydrogen and which contains at least one element from the group IV elements and at least one element from group VII elements is used in the etching gas. At least 86% of an inert gas could be added to the etching gas. Here, the applicants state that this etching gas enables a high etching selectivity and prevents formation of fences.
It is principally desired to provide a method having enhanced properties for reactive ion etching for etching features into a substrate.
It is further desired to provide a method having enhanced properties for reactive ion etching for etching features into a substrate where the etching process has a high degree of anisotropy.
It is further desired to provide a method having enhanced properties for reactive ion etching for etching features into a substrate using halogenated heterocylic hydrocarbons, either alone or in combination with other etching compounds, to provide enhanced properties for reactive ion etching.
It is still further desired to provide a method having enhanced properties for reactive ion etching for etching features into a substrate that optimizes the process by judicious choice of etchant molecule alone.
It is further desired to provide a method having enhanced properties for reactive ion etching for etching features into a substrate where the polymeric deposits that form during the etching process are more reactive and hence more easily chemically removed after the etching process is complete.
Finally, it is desired to provide a method having enhanced properties for reactive ion etching for etching features into a substrate where the presence of the heteroatom makes the source material water-reactive or caustic-reactive and hence easily scrubbable such that emissions may be easily remediated.
The present invention is directed to a method for etching features into a substrate by removing substrate material from selected areas while leaving the substrate substantially unaffected in other areas. The method first includes the steps of providing the substrate to be etched in a process chamber and providing a patterned mask on the substrate as a guide for selective removal of the substrate where the substrate has a mask area and mask-free area. A chemical species of halogenated heterocylic hydrocarbons is introduced into the process chamber and an excitation energy is applied to the process chamber to cause the chemical species to dissociate and form reactive ions and neutral species. An electric potential gradient is maintained in an area adjacent the substrate to impose directionality and anisotropy to the etch.
Preferably, the chemical species includes introducing halogenated heterocylic hydrocarbons alone or in combination with other etching compounds. The halogenated heterocyclic hydrocarbons may be fluorinated heterocyclic hydrocarbons. Additionally, it is preferable that the chemical species includes perfluorinated heteroaromatic amines such as perfluoropyridine, perfluororimidazole, perfluoropyrazine, perfluoropyrimidine, cyanuric fluoride, perfluoropyrrole, perfluoropyrazole, and perfluoropyridazine.
Preferably, the substrate is made from mono- or polycrystalline-silicon, silicon-germanium alloy, gallium arsenide or aluminum oxide and the substrate is coated with one or more thin film layers. Only a topmost layer of the one or more thin films layers may be selectively etched. The substrate coated with one or more films may include a film of silicon oxide, silicon nitride, silicon carbide, silicon oxycarbide, polycrystalline silicon, epitaxial silicon, indium oxide, indium phosphide, silicon-germanium, doped oxide glass, polyimide, or poly(arylene ether).
It is preferable that a pressure in the process chamber is provided that is at or below atmospheric pressure and preferably at or below 150 PA and more preferably between 1 and 100 Pa. The chemical species may be introduced into the process chamber as a vapor and/or with a carrier gas, e.g., helium, neon or nitrogen, and/or with additional etching agents, e.g., CHF3, CF4, C2F6, C3F6, C4F8, C4F6, or C5F8, and/or with a diluent gas (e.g., helium) to dilute and reduce the resonance time of the chemical species in the reactor, and/or with an oxygen-containing gas, e.g. 02, N20 or C02.
The step of applying excitation energy may include applying externally applied radiation, e.g. ultraviolet light, by applying a radio frequency electric field, or by generating the radio frequency electric field by a rapidly varying electric voltage between a substrate support and at least one wall of the chamber or by generating the radio frequency electric field by rapidly varying an electric voltage between a substrate support (i.e. a first electrode) and a second electrode disposed such that the substrate is held between the substrate support and the second electrode.
The step of applying excitation energy may include inducing a rapidly alternating potential in the chamber with an externally situated coil, or using a capacitively coupled plasma reactor, or using an inductively induced plasma reactor, or using an electron cyclotron resonance reactor, or applying an externally generated magnetic field to the etching chamber to help control motion of charged ions in the reaction chamber.
The step of maintaining the electric potential gradient in an area adjacent the substrate may include applying a voltage to a substrate support to force the electric potential gradient to the desired magnitude and polarity or this step may be accomplished by xe2x80x9cself-biasingxe2x80x9d that provide the electric potential gradient with a negative polarity to accelerate positive ions.
The step of maintaining the electric potential gradient may include modulating the electric potential gradient with a frequency between 0.1 kHz and 100 MHz.